The reworking of composite structures frequently requires the localized application of heat. For example, when installing a patch in a rework area of a composite structure, heat must typically be applied to the adhesive at the bondline between the patch and rework area in order to fully cure the adhesive. When applying heat to the patch, the temperature of the bondline must typically be maintained within a relatively narrow temperature range which must be held for an extended period of time until the adhesive is cured. Overheating or under heating the rework area or structure located adjacent to the rework area is generally undesirable during the rework process.
Conventional heating equipment for heating composite structures may include heating blankets comprised of electrically resistive heating elements. A heating blanket may be positioned to heat the patch installed in the rework area. The heating blanket may conductively heat the patch, rework area and/or bondline to the desired temperature. Unfortunately, conventional heating blankets may lack the ability to provide uniform heat at the bondline for several reasons. For example, variations in the construction of conventional heating blankets may result in differential heating across the rework area. In addition, conventional heating blankets may lack the ability to compensate for heat sinks located adjacent to the rework area. Such heat sinks may comprise various elements such as stiffeners, stringers, ribs, bulkheads and other structural members in thermal contact with the structure. Such heat sinks may have relatively large thermal capacities and may therefore draw heat away from certain portions of the rework area while remaining portions of the rework area are continuously heated by the heating blanket. As a result, portions of the rework area that are adjacent to the heat sink may not reach the required adhesive curing temperatures.
Attempts to overcome the non-uniform heating of the rework area due to heat sinks include the use of waxes or other liquefiable materials that are added to an interior of the resistive heating blankets in attempts to distribute heat more uniformly throughout the rework area. In this manner, the wax or liquefiable medium allows for more intimate thermal contact between the heating blanket and the rework area of the composite structure. Unfortunately, the addition of wax increases the overall thickness and stiffness of the heating blanket reducing the ability of the heating blanket to substantially conform to the shape of a rework area in a complexly-curved location of the composite structure. A further issue associated with the use of wax in conventional heating blankets is the risk of contamination of the rework area in the event that the enclosure containing the wax develops a leak.
Further attempts to provide uniform heat distribution using conventional resistive heating blankets include multi-zone blanket systems, feedback loop systems, positive temperature coefficient heating elements, and temperature stabilizing plugs. Unfortunately, the additions of such systems to conventional resistive heating blankets are generally ineffective in providing a substantially uniform temperature without substantial variation across the bondline of the rework area.
As can be seen, there exists a need in the art for a system and method for heating a structure such as a rework area of a composite structure in a manner which maintains a substantially uniform temperature across the rework area. More specifically, there exists a need in the art for a system and method for uniformly heating a composite structure and which accommodates heat drawn from the rework area by heat sinks and other thermal variations located adjacent to the rework area. Furthermore, there exists a need in the art for a system and method for uniformly heating a composite structure in a manner which prevents overheating or under heating of the composite structure. Ideally, such system and method for uniformly heating the composite structure is low in cost and simple in construction.